1. Field of the Invention
The present invention relates to a non-contact system for detecting the level of a liquid metal bath; it may be used in any system where it is desired to measure the distance between an inductive coil and a metal object, whether that object be ferrous or non-ferrous. The invention finds specific application for monitoring the distance between a conductive fluid such as a molten metal bath in a continuous casting mold and the inductive sensor coil.
An inductive, non-contact distance measurement system for liquid metal bath levels is described, for example, in Yamada et al., U.S. Pat. No. 4,030,027, issued June 14, 1977. In that patent, an inductive coil is employed in the feedback circuit of an operational amplifier having positive and negative input terminals, the negative input terminal is connected to an oscillator and the positive input terminal is connected to the aforesaid feedback circuit. The amplified differential output signal from the amplifier, comprising the difference between the oscillatory signal applied to the differential amplifier and the signal applied to the differential amplifier which varies as a function of the impedance of the detecting coil corresponding to the distance between the detecting coil and the metallic body, are a measure of the distance between the two. At least one of the parameters of open-loop gain of the differential amplifier and the amount of feedback in the feedback circuit are predetermined so as to liberalize at least for a predetermined measuring range the output characteristics of the differential amplifier resulting from the variation of the detecting coil impedance caused by the variation of the distance.
Other systems of this type have attempted to provide a liberalizing change in the detector circuit for changes in the performance of the detector coil with the use of bridges, phase-angle correction circuits, and the like. These earlier circuits incorporated the linearalizing correction upstream of the first operational amplifier or in the feedback path, exemplified by the aforesaid U.S. Pat. No. 4,030,027.
In co-pending application Ser. No. 661,726 filed Oct. 17, 1984, now U.S. Pat. No. 4,956,606, there is disclosed a non-contact, inductive measuring system for monitoring the distance between a metallic object, such as the top surface of molten, liquid metal bath and a detection coil capable of accurately measuring small separation distances as well as extended separation distances without moving or repositioning the coil.
The detecting coil is surrounded by heat-insulative sleeve within a sensor shell. An open end of the shell is fitted to a air flow ducts such that coolant air can pass centrally to the coil and along an annular discharge flow space to the air discharge duct.
The coil is electrically coupled in a series arrangement with a capacitor to form a series resonant, L-C circuit. The L-C circuit is connected to an input terminal of an amplifier and an oscillator is connected to a second input to the amplifier. The frequency of the oscillator is offset with respect to the resonant frequency of the L-C circuit, whereby the output signal of amplifier is essentially linear. This signal, the magnitude of which varies in proportion to the separation distance between the coil and a metallic object, is supplied to temperature compensation circuit which functions to take into account variations of the output signal due to fluctuations of the power supply voltage, the temperature of the environment of the electronic circuit, and the temperature of the coil. The output signal of temperature compensation circuit is supplied to readout.